1. Field of the Invention
The present invention relates to a cylinder head of an engine.
2. Description of the Related Art
In general, a cylinder head of an engine is built on an upper side of a cylinder block with head bolts, and intake valves for opening and closing intake ports of combustion chambers, exhaust valves for opening and closing exhaust ports of the combustion chambers, a camshaft and rocker arms which actuate the intake and exhaust valves to open and close the intake and exhaust ports, a rocker arm shaft which supports the rocker arms and the like are provided on the cylinder head. The rotation of a crankshaft is transmitted to the camshaft via timing gears or a timing belt.
A plurality of camshaft support portions for supporting the camshaft rotatably and a plurality of rocker arm shaft support portions for supporting the rocker arm shaft rotatably are provided on the cylinder head. Specifically, these camshaft support portions and rocker arm support portions are each formed into a concave shape (a semi-circular shape) for accommodating the shafts and their journal portions for support. These shaft support portions are disposed along axial directions of the shafts.
On the other hand, as has been described above, the cylinder head is fixed to the cylinder block with the head bolts. A plurality of head bolt boss portions are formed on an upper surface of the bottom wall portion of the cylinder head in positions where the head bolts are passed.
Incidentally, a space is necessary on the periphery of the head bolt boss portion for a head bolt tightening tool to be inserted. Because of this, a relatively wide space is secured on the periphery of the head bolt boss portion.
The cylinder head tends to be enlarged in size as the camshaft support portions, the rocker arm shaft support portions and the head bolt boss portions are formed in the interior thereof and the spaces need to be secured on the peripheries of the head bolt boss portions as described above. However, it is required from the viewpoint of reducing the overall size of the engine to arrange the shaft support portions and the head bolt boss portions in such an appropriate way as to suppress the enlargement of the cylinder head.
For example, Japanese Patent No. 3263118 can be raised as disclosing a construction which can realize the reduction in size of the cylinder head. In Japanese Patent No. 3263118, there is proposed a devised positional relationship between supporting structures and head bolts.
In Japanese Patent No. 3263118, lower bearing members are installed on and between left and right support walls which are provided on an upper surface of a cylinder head in such a manner as to be erected therefrom. Cam caps are installed on the lower bearing members, and a camshaft is supported between the lower bearing members and the cam caps. Head bolts are disposed between the support walls, and spaces are secured between the head bolts and the support walls so that the head bolts are inserted and removed therethrough. The cylinder head is fastened to a cylinder block using the head bolts, and thereafter, the lower bearing members are built on and between the support walls.
In the construction disclosed in Japanese Patent No. 3263118, the lower bearing members and the head bolts are overlapped each other in a height direction, whereby the reduction in size of the cylinder head is realized.
On the other hand, in addition to the reduction in its size, it is required for the cylinder head to ensure a sufficient rigidity, and to make this happen, reinforcement portions such as ribs are provided. However, in the event that such high rigidity is attempted to be realized in the construction disclosed in Japanese Patent No. 3263118, separately from the support walls which are disposed in consideration of ensuring the head bolt insertion and removal spaces, reinforcement portions such as ribs need to be disposed within the cylinder head. It is considered that the disposition of such ribs in the cylinder head makes it difficult to reduce the size of the engine to a sufficiently small level.
In addition, since the structures for supporting the camshaft on the cylinder head are provided on a bottom wall of the cylinder head in such manner as to be erected integrally therefrom, it is considered that the supporting structures themselves function to reinforce the cylinder head to make it highly rigid. However, in the camshaft supporting structures disclosed in Japanese Patent No. 3263118, since the construction is adopted in which the support wall and the lower bearing member are made up of the separate parts, the strength is reduced compared to an integral construction, and the function as the reinforcement portion of the cylinder head is reduced.
Furthermore, in the event that the support wall and the lower bearing member are made up of the separate parts as with the construction disclosed in Japanese Patent No. 3263118, an increase in production costs is called for due to an increase in the number of parts involved.